COP v3.0:durability;

Durability 

Corrosion (B2-Durability) covers considerations for continued performance of roof and wall cladding over the building lifecycle.
Key topics include:

  • Material performance and coatings.
  • Corrosion: evironmetal categories and special climates.
  • Material compatibility — both "in contact with" and "runoff onto".
  • Maintenance for prevention and remediation of corrosion.

4.1 NZBC Clause B2 (Extract) 

Source: New Zealand Building Code, Clause B2 Durability

4.1.1 B2 Objective 

The objective of this provision is to ensure that a building will continue to satisfy the other objectives of this code throughout its life.

4.1.2 B2 Functional Requirements 

Building materials, components and construction methods shall be sufficiently durable to ensure that the building, without reconstruction or major renovation, satisfies the other functional requirements of the NZBC throughout the life of the building

4.1.3 Performance Requirements 

Building elements must, with only normal maintenance, continue to satisfy the performance requirements of the NZBC for the lesser of the specified intended life of the building, if stated, or:

  1. The life of the building, being not less than 50 years, if:
    • those building elements (including floors, walls, and fixings) provide structural stability to the building; or
    • those building elements are difficult to access or replace; or
    • failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.
  2. 15 years if:
    • those building elements (including the building envelope, exposed plumbing in the subfloor space, and in-built chimneys and flues) are moderately difficult to access or replace; or
    • failure of those building elements to comply with the building code would go undetected during normal use of the building, but would be easily detected during normal maintenance.
  3. 5 years if:
    • the building elements (including services, linings, renewable protective coatings, and fixtures) are easy to access and replace; and
    • failure of those building elements to comply with the building code would be easily detected during normal use of the building.

Individual building elements which are components of a building system and are difficult to access or replace must either:

  • all have the same durability; or
  • be installed in a manner that permits the replacement of building elements of lesser durability without removing building elements that have greater durability and are not specifically designed for removal and replacement.

4.1.3.1 Compliance 

NZBC Clause B2, Durability, requires fifteen years to perforation for claddings easily accessed for replacement. Fifteen years is also required for internal gutters and downpipes, and five years for external gutters.

NZBC B2 requires 50 years’ durability for flashings that require the removal of cladding above to be replaced, while table 20 of NZBC E2/AS1 only requires 15 years’ durability for such flashings. The COP recommends the higher figure as good trade practice and in many cases, lower life-cycle costing.

Generally, higher durability than the minimum requirements can be achieved by using materials and methods outlined in this COP, with no maintenance of coatings other than washing areas which are not naturally washed by rain. Elements more difficult to replace, or to access for maintenance, should be constructed of more durable material.

Normal Maintenance means work recognised as being necessary to achieve the expected durability of a given building element.

According to B2/AS1, normal maintenance may include:

  • Washing down surfaces subject to wind-driven salt spray and contaminants.
  • Re-coating protective surfaces.
  • Replacing sealant seals and gaskets in joints.

Although roof or wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for flashings. Flashings might be embedded in plaster or behind other building elements, making them hard to replace without removing cladding or other building features such as windows.

Cladding material may be described as hidden, sheltered or exposed. Some flashings may have sections falling into all these categories, in which case the worst case (sheltered) should prevail in material selection.

All metal roof and wall cladding and accessories must be designed and installed to comply with the durability requirements of the NZBC. NZBC requirements relate to performance, however, and do not necessarily relate to aesthetics or cost of replacement. Any pre-painted cladding will change colour over time, and partial replacement would be visible. The roof cladding could have deteriorated, although not perforated, within 15 years and still comply with the NZBC, but customer expectations may not be met.

Good design, correct selection of materials, and good installation and maintenance practices are required to achieve optimum product lifespan. 

4.2 Metal Corrosion 

Corrosion is the process by which something erodes because of a chemical reaction.

Metal corrosion is a reaction of metal with its environment that causes measurable alteration and is part of metal's inherent tendency to revert to an original, more stable form. The red rusting of iron and steel is a visible example of corrosion and other examples include the weathering of copper and the oxidation of aluminium and zinc.

Corrosion can only happen in the presence of an electrolyte, e.g., water. The occurrence of salt (or other contaminants) in the water increases the conductivity of the electrolyte and therefore greatly increases the reaction rate.

Salt contamination will also affect the time of wetness.  On a clean surface, water vapour will condense at 100% relative humidity, on a salt-covered surface, a wet film can be formed at a relative humidity level of 75% and more.

Corrosion can also be the result of direct contact with another metal or substance, or the result of run-off from incompatible surfaces or fall-out of corrosive particles. Time of wetness, presence or lack of oxygen, and atmospheric contaminants greatly affect the rate of corrosion.

Differences in electrical potential on the surface of corroding metal create microscopic cells comprising cathodes and anodes. In the case of iron, the positively charged electrons in the anode react with the negatively charged hydroxyl ions in the electrolyte to form iron oxide on the anode. Similar reactions occur with other metals. Polarisation changes on the surface cause anodic areas to become cathodic and vice versa, so that over time the rate of corrosion is relatively uniform over the surface.

The build-up of debris on a cladding surface will promote corrosion. The salts in the debris react with the cladding each time they are wetted, and the deposits themselves impede surface-drying, increasing the time of wetness.

4.3 Metal Performance 

To understand metal performance in any specific environment, the unique properties of each metal should be considered in conjunction with other metals it is used with.

All metals react differently to the atmosphere and to any contaminants that come into contact with their surface by rain, wind or condensation. The indicator of acidity or alkalinity is pH, measured from 0 – 14; acidity/alkalinity is an important corrosion factor. pH 7 is neutral, below 7 is more acidic, above 7 is more alkaline.

 

 

As can be seen from the graph above, zinc performs better in alkaline environments, and aluminium performs better in acidic environments.

Aluminium-zinc coatings should be avoided in buildings such as closed animal shelters or fertiliser storage sheds, where alkalinity may be high.

4.4 Sacrificial and Barrier Protection 

The zinc and aluminium families of metallic coatings protect the steel base from corrosion in two different ways:

  • Zinc predominant coatings protect the substrate primarily by offering sacrificial protection.
  • Aluminium predominant coatings primarily offer barrier protection.

4.4.1 Sacrificial Protection 

Zinc is more electrically active than steel. By coating steel with zinc, or a zinc-rich product, the zinc becomes the anode for the steel. The steel then becomes the cathode and does not react with the electrolyte. The process is known as cathodic protection.

This protective effect occurs even when there is a small area of steel exposed directly to the electrolyte, such as a cut sheet edge, drill hole or scratch.

While the zinc reacts in preference to the steel, it does so at a slower rate. In normal environmental conditions, the zinc-oxide layer that initially forms on the surface of the zinc combines with carbon dioxide in the atmosphere to form zinc carbonate. That creates a sealed layer with excellent adhesion, and as zinc carbonate has very low solubility, reaction with the electrolyte slows even more.

4.4.2 Barrier Protection 

Barrier protection works primarily by providing a physical barrier between the atmosphere and the steel substrate.

The surface of aluminium-dominant coatings is initially very active, but it quickly forms an inert aluminium-oxide film when exposed to normal atmospheric conditions. Aluminium dominant coatings on steel mainly provide barrier protection as the aluminium, having formed an oxide surface, ceases to offer substantial sacrificial protection.

The exposed edges of barrier protected cladding should not be in contact with corrosive surfaces. See 4.9.4 Compatibility Table

4.5 The Environment 

4.5.1 Atmosphere 

The durability performance of metal roof and wall cladding depends on the macro- and microclimates, airborne contaminants, and the material itself.

The macroclimate is the general environmental category where the building is situated.

The microclimate relates to the exact location of the building and the design or position on the roof or wall. Microclimate influences include geothermal fumaroles, rain sheltering, topography and ground roughness, prolonged wetness, and exclusion of oxygen. Internal microclimates can also occur as result of the particular use of the building.
Contaminants and pollutants are corrosive influences which can affect the cladding. These can include fertiliser, soil, leaf fall-out, exhaust fumes, industrial fumes, bird droppings and the build-up of debris. Influences such as chlorides near the sea, geothermal hydrogen sulphide (H2S) or man-made gases such as sulphur dioxide (SO2) accelerate the corrosion rate by increasing the conductivity of the electrolyte and changing its pH value.
Rain provides the moisture that acts as the electrolyte in corrosion cells. Rain varies in pH because it picks up various contaminants from the pollutants in the atmosphere. Acid rain can happen in geothermal areas due to the presence of hydrogen sulphide in the atmosphere.
At 0°C metal corrosion is minimal, because colder temperatures slow the reaction. The corrosion rate of some metals doubles with every 10°C rise in temperature given the same time of wetness and environmental conditions. However, in dry, warm environments the time of wetness is decreased by faster drying times, which has the opposite effect.

Designers should be aware of macro- and microclimates and the degree of contamination. They should design their building and select materials considering a combination of all these factors.

4.5.2 Sea Spray 

The major contributor to metal corrosion in New Zealand is sea spray. Sea spray contains a mixture of salts consisting of 2.5 to 4% sodium chloride and small quantities of magnesium, calcium and potassium chloride. These salts make water far more electrically conductive.

Sea spray, evaporation, and infrequent rain increase salt concentrations on exterior surfaces, particularly when it accumulates in unwashed areas.

 

4.5.2A Airbone Salt from Sea Spray.

Onshore winds, big swells, wide generation zone and rugged coastline make ideal conditions for the production of salt aerosol.

 

The distance airborne salt is carried inland varies significantly with local wind patterns. Salt deposits have been measured as far inland as Lake Taupo in the North Island. Geographic or man-made obstructions, such as trees or buildings, slow air velocity and allow the air to discharge some of its salt burden, which can make the environment less aggressive. Conversely, where there are few impediments to the free flow of air, severe marine influence can extend well inland.

In high humidity levels, or when wetted by condensation, marine salts absorb water and form a chloride solution. Therefore, the effect of salt spray is greatest in unwashed areas, where salts can accumulate over time.

Where the ends of roof cladding are exposed to contaminants such as sea salt or industrial pollutants, it is good practice to provide an over flashing which discharges into the gutter or spouting. (See 8.4.4.4A Gutter-Eaves Flashing.)

  • It gives a measure of protection to the underside of the roof cladding and the underlay.
  • It provides support for the roofing underlay which is subject to damage from wind and UV.
  • When using PVC spouting, there is a gap between the spouting and the fascia caused by the thickness of the brackets. In coastal locations where the ends of roof cladding are exposed, this unwashed area becomes susceptible to corrosion. A gutter apron can minimise this risk.
  • If there is no spouting or it has a low front.
  • In severe environments, wind can drive contaminants up the ribs of exposed ends of roof cladding. Metal scriber flashings or filler blocks can be used to prevent or inhibit ventilation.

The over flashing should extend 50 mm into the gutter, and the underlay finishes on the down-slope of the flashing. If there is no over-flashing to the gutter, the underlay should be extended into the gutter by a minimum of 20 mm.

In some cases, the over flashing becomes a sacrificial flashing which can extend the life of the cladding. In such circumstances, the COP recommends making the flashing from aluminium.

4.6 Environmental Categories 

Suppliers of pre-painted metal offer alternative products for different environments, using different metallic coatings, paint systems, paint thickness and metals. The designer or the roof cladding contractor should carefully assess and evaluate these options to comply with the NZBC.

The boundaries of different corrosion zones are difficult to define because many factors determine the corrosivity of a particular location. The designer should choose the appropriate materials for the location, which meet the minimum durability requirements of the NZBC and satisfy customer expectations.

4.6.1 Assessment of Marine Environments 

Wind is responsible for the salinity present in marine atmospheres. The wind picks up particles of salt from breaking waves and can carry them inland. The quantity of salt aerosol entrained by the wind is affected by many factors, such as wind strength, wave height, the width of the generation zone, and the contours of the seabed and coastline. These factors along with the persistence of the wind from a given quarter determine the corrosivity of a shoreline.

While salt deposits are measurably present in inland areas such as Taupo, the main effect of marine atmospheres reaches just a few hundred metres from the shore.  Particles of salt in the air deposit on adjacent surfaces through gravity and contact; the rate at which deposits settle is affected by the roughness of the ground that the salt-laden air passes over. Obstacles such as trees slow the wind down, increasing the rate of gravitational deposit, and bringing the salt aerosol in more contact with surfaces on which they can deposit.

On the other hand, open flat land and natural “wind tunnels” can allow quite high concentrations of salt to travel several hundred metres inland.

A site’s location, relative to the sea or marine inlets, is a common method used to assess the corrosivity of a location. The distance from salt water for a given Zone varies with the location, depending on the prevailing winds and roughness of water in those areas, as well as the evenness of the terrain it passes over.

Where environmental Zones overlap, a site-specific evaluation may help define the category into which it best fits.  Visual evidence of corrosion on adjacent metal surfaces may be present, ground roughness can be assessed, industrial influences can be evaluated and data about the persistence of onshore winds can be obtained from NIWA.

More local factors that affect the corrosivity of a specific location include:

  • Overhanging shade increases the time of wetness of a structure and corrosion rate.
  • High levels of water roughness such as caused by strong tidal flow against the wind direction, as is often experienced in areas such as Cook Strait, increases salt spray.
  • Surfaces not receiving regular and effective rain washing or sufficient manual washing may experience corrosion rates two to three times that of cleaned surfaces.

There are many ways of more accurately determining the actual corrosivity of a given location. The most commonly accepted method as outlined in ISO 9223 is measuring first-year corrosion rate of different metals: mild steel (MS), zinc, aluminium and copper. The COP uses the first-year corrosion rate of mild steel as the most relevant and reliable indicator of a location’s corrosivity.

The names given by different Standards for given corrosion zones vary. The Corrosion Zones in the Code of Practice are similar to those published in NZS 3604:2011 except that:

  • the COP makes a distinction between Harbours, West Coast, and East Coast shorelines, and
  • NZS 3604 Zone D (High) is further broken down into E (Very High) and F (Extreme Marine) because, in NZS 3604 Zone D, the first-year mild steel corrosion rate can vary from 200 g/m2 to 1000 g/m2.

 

4.6.1A Corrosion Zone Categorisation and First year Mild Steel Corrosion Rate (g/m²)

NZS3604Code of PracticeDescriptionMS Corrosion Rate (g/m²)
BA  (Mild)Far inland, with no industrial pollution or thermal activity, or dry internal. This condition is not commonly found externally in New Zealand.1 – 10
B  (Moderate Inland)Most dry rural areas in New Zealand, 50 km from the coast, are in this category. It can extend closer to the coastline of sheltered water in low rainfall areas.10 – 80
CC  (Moderate Marine)This category covers area of low marine influence. It can extend from 50 km inland to within 1 – 1.5 km of west coast beaches, or be in the immediate vicinity of calm estuaries.80 – 200
DD  (Severe Marine)In this category, marine influences are frequently apparent. Its proximity to the coast is determined by the roughness of the water, prevailing winds, ground roughness and sheltering.200 – 400
E  (Very Severe Marine)In this category, the structure is normally exposed and marine influences are almost constantly apparent.400 – 650
F  (Extreme Marine)This category is rare in a building site. It would be an exposed location very close to breaking surf.650 – 1 000

4.6.3 Material Selection 

Note: this is the minimal requirement to achieve compliance with NZBC Clause B2-Durabilty.  Meeting the minimum requirements of NZBC clause B2 Durability does not necessarily represent optimal product selection. In a transition zone, it may be more cost effective over the life cycle of the building, and for meeting customer expectations, to choose a more durable option.

4.6.3A Material Selection : Exposed Roofs and flashing

Durability Required : 15 years

Marine ZoneExposed Fastener
Class (minimum)
Acceptable Materials
***As defined by AS/NZS 2728.
B:
Moderate Inland
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
  • AZ 150 coated steel
  • Galvanised steel Z 450
C:
Moderate Marine
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
  • AZ 150 coated steel
  • Galvanised steel Z 450
D:
Severe Marine
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
E:
Very Severe Marine
C5
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
F:
Extreme Marine
C6
  • Aluminium
  • Pre-painted aluminium
 
Materials accepted by NZMRM as complying with coating types include:
  • Painted steel Type 4:
    Colorsteel® Endura®, Colorcote® ZinaCore™
  • Painted steel Type 6:
    Colorsteel® Maxx®, Colorcote® MagnaFlow™, Colorcote® MagnaFlow X™

 

4.6.3B Material Selection : Walls*, fascias and sheltered roofs and flashings

Durability Required : 15 years

Marine ZoneExposed Fastener
Class (minimum)
Acceptable Materials
*The practicality of carrying out regular maintenance, and difficulty of replacement, should also be considered when considering wall cladding material options.
***As defined by AS/NZS 2728.
B:
Moderate Inland
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
  • AZ 150 coated steel
  • Galvanised steel Z 450
C:
Moderate Marine
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
  • AZ 150 coated steel
D:
Severe Marine
C4
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
E:
Very Severe Marine
C5
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
F:
Extreme Marine
C6
  • Aluminium
  • Pre-painted aluminium
 
Materials accepted by NZMRM as complying with coating types include:
  • Painted steel Type 4:
    Colorsteel® Endura®, Colorcote® ZinaCore™
  • Painted steel Type 6:
    Colorsteel® Maxx®, Colorcote® MagnaFlow™, Colorcote® MagnaFlow X™

 

4.6.3C Material Selection : Flashings Behind Cladding

Durability Required : 50 years

Marine ZoneAcceptable Materials
**Stainless steel must not be in wet contact with metallic coated steel, plain or painted.
***As defined by AS/NZS 2728.
B:
Moderate Inland
  • Stainless Steel**
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
  • Pre-painted steel Type 4***
  • AZ 150 coated steel
  • Galvanised steel Z 450
C:
Moderate Marine
  • Stainless steel**
  • Aluminium
  • Pre-painted aluminium
  • Pre-painted steel Type 6***
D:
Severe Marine
  • Stainless steel**
  • Aluminium
  • Pre-painted aluminium
E:
Very Severe Marine
  • Stainless steel**
  • Aluminium
  • Pre-painted aluminium
F:
Extreme Marine
  • Stainless steel**
  • Aluminium
  • Pre-painted aluminium
 
Materials accepted by NZMRM as complying with coating types include:
  • Painted steel Type 4:
    Colorsteel® Endura®, Colorcote® ZinaCore™
  • Painted steel Type 6:
    Colorsteel® Maxx®, Colorcote® MagnaFlow™, Colorcote® MagnaFlow X™

4.7 Special Climates 

In areas where humidity or local conditions create an increased likelihood of corrosion, special consideration should be given to the specification and use of metal roof and wall cladding and accessories.

4.7.1 West Coast, South Island, South Coast, North Island 

The West Coast is characterised by high rainfall and a very severe coastal environment between the sea and the Southern Alps; many households have coal burning fires that produce sulphur dioxide that is detrimental to metals.The combination of these factors means that either a shorter performance life should be accepted or the use of more durable metals and coatings considered.

The North Island’s south coast not only has strong onshore prevailing winds, but strong current flows increase wave action and the amount of salt-laden air, creating a particularly harsh marine environment.

4.7.2 Geothermal 

Buildings within 50 m of a geothermal fumarole are considered to have a geothermal microclimate, which causes increased corrosion due to higher humidity levels combined with hydrogen sulphide.

Highly active geothermal areas, such as much of Rotorua, are considered geothermal, even in the absence of a local fumarole.

4.7.3 Internal Environments 

Corrosive internal environments with high humidity, causing condensation, and pollutants generated within the building can also affect neighbouring buildings. These include:

  • covered swimming pools;
  • fertiliser works;
  • meat works;
  • animal sheds or shelters;
  • pulp and paper manufacturing; and
  • vehicle exhaust fumes.

4.7.4 Fossil Fuel Residue 

Sulphur dioxide develops when burning fossil fuels. After oxidation and reaction with water it forms sulphuric acid (H2SO4) that can contribute considerably to the atmospheric corrosion of zinc and steel.

Burning resinous woods, CCA-treated timber, low-grade coal or oils with a high sulphur content can increase the fall-out deposit or condensate from flue gas.

Exhaust fans can cause similar problems when corrosive gases are not filtered at their source.

4.8 Paint Durability 

New Zealand has a harsh level of ultraviolet light. Paint formulations that are successful overseas have sometimes been found lacking colour fastness in the Australasian environment. Proprietary pre-painted cladding products should give durability protection and keep its appearance for at least 15 years, and at the end of that period still have the anti-corrosive primers intact and present a good substrate for over-painting.

The performance of all paint coatings, however, can be affected by avoidable outside influences.

4.8.1 Touch-up Paint 

Colour match paint is sold in pre-colour matching accessories, such as soft edging or brackets, to match the pre-painted cladding. It should only be pre-applied before installation and should not be used to repair minor scratches and blemishes. These paints may closely match the pre-painted surface initially, but being an air drying acrylic, it is likely to weather differently to the pre-applied coating and cause unsightly blotches.

 

 

4.8.2 Sunscreen Lotions 

Sun screen lotions containing semi conducting metal oxides, such as titanium dioxide and zinc oxide, will cause discoloration of painted surfaces over time. There is no cure for such damage so contact of such chemicals with pre-painted cladding must be avoided.

 

 

4.8.3 Graffiti Removal 

Most Graffiti removal processes involve the use of strong solvents, abrasion, or extreme temperatures. It is generally difficult to remove graffiti without compromising an existing organic finish; the normal remedy is to overpaint as soon as possible. On a weathered pre-painted finish this can generally be achieved by washing and rinsing the surface to remove dirt and other contaminants, then applying two coats of acrylic paint.

It should be noted that such overpaint cannot over time, be expected to match the appearance of existing pre-applied finishes.

4.8.4 Lichen 

Warm temperatures, dust, and rainfall can create an environment for lichens to flourish. Over time the root structures of lichen may infiltrate a painted surface and cause permanent damage.

Lichen growths retain moisture and, therefore, increase the time of wetness. They are fed by nutrients in the atmosphere and tend to occur more commonly in moist and unpolluted environments. Where lichen growth is present, it should be removed. See 16 Maintenance .

Physical removal is difficult to achieve completely, and recolonisation is usually rapid. Chemical treatment is recommended. New Zealand Steel publishes a formula for batching a 2% sodium Hypochlorite solution to be used for this.

 

 

4.9 Compatibility 

Materials comprising the building envelope should not be considered in isolation, as their performance can be affected by contact with or run-off from other materials.

This reaction is caused by either their relative places in the electro-chemical series or by the mineral composition of their surface moisture.

4.9.1 Disimilar Metals 

A component which may appear suitable may prove unsatisfactory in service because it is incompatible with another material or substance in contact with it.

This incompatibility can occur when the metals are in electrolytic contact or when water from one metallic surface discharges onto another. When a noble metal dissolves in water and flows over a less noble one, the more noble metal deposits on the less noble metal and create corrosion conditions.

4.9.1A Galvanic Series

Galvanic Series
MagnesiumActive (Anode)
Zinc 
Galvanised Steel 
Aluminium 
Mild Steel 
Cast Iron 
Lead 
Brass 
Copper 
Bronze 
Monel 
Nickel (passive) 
Stainless Steel 304 (passive) 
Stainless Steel 316 (passive) 
Silver 
Titanium 
Gold 
Graphite 
PlatinumNoble (Cathode)
The similarity of metals is indicated by their relative position in the galvanic series. The more dissimilar the metals, the greater the corrosion potential in a galvanic circuit

 

Generally, run-off from metals higher on the table to those lower will not cause corrosion, but run-off in the other direction may do so.

Metals such as aluminium, stainless steel, and Zincalume® form an inert surface that does not produce soluble salts and run-off from them will not result in dissimilar metal corrosion. However, because these surfaces are inert, potential for run-off to create inert catchment corrosion on unpainted zinc or galvanised steel must be considered.

 

4.9.1B Lead in Contact with AZ

Lead in contact with coated or uncoated AZ will cause premature corrosion.

 

4.9.1C Cladding in Contact with Stainless Steel Rivet

Stainless rivets will cause corrosion to Z AZ and ZA coated products.

Where the use of dissimilar metals is unavoidable, a non-absorbent inert material can be used as an electrolytic separator. Long-term corrosion resistance depends on the separation remaining effective.

Examples of separation materials are inert plastic tapes, polythene or silicone sealant, and in the case of fasteners an EPDM sealing washer.

Where gutters and spouting are made from metals incompatible with roof cladding, there can be contamination from splashing or immersion of the roof sheet ends into the poorly drained gutter. Special provisions—such as painting the inside of a copper spouting, or an apron flashing—should be made when using copper or lead gutters and spouting with coated steel roof cladding. See 14.13 Fasteners. The top of copper spouting should not touch coated steel cladding, tiles or flashings.

 

4.9.2 The Electrochemical or Galvanic Series 

The electrochemical tables or galvanic series scales, often quoted in technical literature as a measure of corrosion, show the electro-potential between pure metals, not between their oxides, carbonates or chlorides.

Although theoretically correct, these tables can give a misleading indication of the performance of different materials in contact.

This series only applies to pure metal. Under certain conditions, some metals react with the environment or chemicals to form a passive surface, which renders them less active, so that any ranking can be misleading. "Passivity" becomes an important phenomenon in controlling corrosion rates.

However, the Electrochemical series table is still a useful indicator of electrode potential. The further apart two metals are in the electrochemical series, the greater the potential difference between them.

Metals termed anodic, active, negative, or less noble corrode in preference to metals that are deemed more cathodic, noble, positive or less active. The less noble metal becomes the anode and is subject to corrosion. The greater the potential difference, the more corrosion there will be of the less noble metal; i.e., on the anode.

The difference in nobility is why zinc can protect a steel substrate.

Different electrolytes can lead to different rankings, and metal alloys may display more than one potential than that which applies to their "active" state.

The exposed surface ratio of anode and cathode determines the rate of dissimilar metal corrosion. For instance, if a fastener which has a small surface compared to the cladding becomes the anode, its current density will be high, and the fastener will corrode quickly; e.g., an aluminium rivet in a copper sheet. When the opposite is the case, the effect is not so great.

The 4.9.2A Electrochemical table shows zinc is more active than steel. Contact between steel and zinc, in the presence of moisture, will cause the zinc to corrode or sacrifice itself, to protect the steel.

 

 

4.9.3 Compatibility with Non-Metallic Substances 

Timber is generally acidic, although some timbers—such as cedar—are more acidic than others. The interaction between preservative treated timber and metal depends on the moisture content of the timber, the time of wetness and the type of treatment. The corrosion rates of metal in contact with wet CCA-treated timber and with untreated radiata are similar.

Neither AZ coated steel nor aluminium should be used as a flashing embedded in concrete or masonry.

4.9.3A Butyl Rubber in Wet Contact with Cladding

Butyl Rubber contact
Butyl rubber in wet contact with coated aluminium and steel has been found to accelerate corrosion.

4.9.4 Compatibility Table 

The compatibility table should be regarded as indicative only, due to the many permutations of the environment, the amount of moisture present and the relative size of the components.

The indicator for “use with caution in moderate environments” should be interpreted as a warning that it could be unsuitable when there is a risk of continued moisture or other contaminants.

 

4.9.4A Interactive Material Compatibility Tool

This online tool interactively provides an interpretation of the information in the 4.9.4B Material Compatibility Table.  Simply select the two materials in use to view compatibility.

The Code of Practice Online provides an interactive tool to interpretation of the information in the 4.9.4B Material Compatibility Table Table, by simply selecting the two materials in use to view compatibility. This tool is only available online at www.metalroofing.org.nz/cop/durability/compatibility#compatibility-table...

 

 

4.9.4B Material Compatibility Table

Material water flows from
Aluminium
pre-painted aluminium
AZ coated steel
zinc or zinc coated steel
pre-painted AZ steel
Copper/brass
Stainless steel
Lead
Plastic/glass
concrete/plaster wet
concrete/plaster dry
Wet timber
Cedar
Butyl rubber
AluminiumContactYYYYYNCNYNCNNC
 Run ontoYYYNYYYYYYYYYY
pre-painted aluminiumContactYYYYYNNNYNCNNN
 Run ontoYYYNYYYYYYYYYY
AZ coated steelContactYYYYYNNNYNCNNN
Zincalume ®Run ontoYYYNYYYYYYYYYY
Zinc or Zinc coated steelContactYYYYYNCYYYYNNN
(galvanised - Z)Run ontoYYYYYYYYYYYYYY
pre-painted AZ steelContactYYYYYNNNYNYNNN
 Run ontoYYYNYYYYYYYYYY
Copper/brass*ContactNNNNNYYCYCYYYY
 Run ontoNNNNNYYCYYYYYY
Stainless steelContactCNNCNYYYYYYYYY
 Run ontoYYYYYYYYYYYYYY
Lead*ContactNNNYNCYYYYYNYY
 Run ontoCNNYNYYYNCCYYY
Plastic/glassContactYYYCYYYYYCYYYY
 Run ontoYYYNYYYYYYYYYY
Concrete/plaster wet*ContactNNNCNCYYYYYYYY
 Run ontoNCNYNYYYYYYYYY
Concrete/plaster dryContactCYYYYYYYYYYYYY
 Run ontoCYYYYYYYYYYYYY
Wet timberContactNNNNNCCNYYYYYY
 Run ontoYYYYYYYYYYYYYY
Steel*ContactCNCNCNNCYCYNNY
 Run ontoYYYYYYYYYYYYYY
Cedar*ContactNNNNNYYYYYYYYY
 Run ontoYYCCYYYYYYYYYY
Butyl rubberContactCNNNNYYYYYYYYY
 Run ontoCCNNCYYYYYYYYY

 

4.9.4C Material Compatibility Key

YSuitable
NNot suitable
CMay need separation. Use with caution in severe or moist environments
*May cause staining, but not corrosion
Note:
  • Runoff and contact effects may vary according to the relative size/area of the two materials.
  • Most incompatible materials will not react if moisture can be eliminated from area of contact.
  • Use with caution - may mean separation required, or unsuitable in severe environments, or when in wet contact.
  • Wet concrete includes uncured concrete, fibre-cement, or within plaster walls.
  • Dry concrete includes cured concrete not exposed to rain.

 

 

4.10 Other Causes of Corrosion 

4.10.1 Unwashed Areas 

Areas on a building that seldom receive rain-washing gather salt, dust and other contaminants. When condensation, dew or humidity moistens these particles, they react with metal cladding. The reaction is often noticeable as a white zinc corrosion effect, which will precede more serious corrosion.

An unwashed or sheltered area is any surface that is above or inside a line drawn at 45° to any weathertight overhang. Such areas require special consideration, particularly in severe environments.

Unwashed areas include: unlined soffits, roof overhangs, canopies, sheltered walls, and the upper part of garage doors.

Wall claddings receive less effective rain washing than roofs, and may be harder to maintain or replace, so materials for wall cladding should be selected accordingly.

 

 

 

 

 

 

 

 

4.10.2 Poultice Corrosion 

Poultice corrosion or 'under deposit corrosion' is caused when a collection of fine dust — e.g., from earthworks and quarries, or sawdust and shavings from timber processing plants — collects in crevices behind laps or flashings.

These collections increase the time of wetness and retain contaminants.

Bird droppings can contain highly corrosive and adhesive materials that will affect metallic cladding. Birds can also deposit other injurious material, e'g', fish frames and chicken bones.

4.10.3 Solar Collectors and Other Roof Mounted Structures 

Solar energy collectors, Heating Ventilation and Air-conditioning (HVAC) systems may be attached directly to the surface of profiled metal roof cladding or mounted on a frame above the roof plane.

For direct fastened solar energy collectors, a complete seal between the photovoltaic (PV) cells and the cladding is essential to prevent water ingress resulting in wet storage corrosion.

Rail mounted systems for solar energy collectors can be installed in a similar manner to HVAC systems and other roof mounted structures. Units may be laid parallel to the roof or oriented at an angle for greater efficiency and easier roof maintenance.

Support frames installed above the roof must have sufficient gap underneath

  • to assist with self-cleaning,
  • limit the build-up of leaves and other debris,
  • provide access for inspection and cleaning, and
  • allow air movement to quickly dry areas under the panel.

The minimum recommended gap is 100 mm from the pan, and 20 mm from the crest of a profile. A greater clearance may be required for larger installations or where greater debris deposits are expected.

The support structure may be connected to the rib of clip fixed products; or supported by the rib and attached through the profile to the purlins. The designer must ensure that the design of the roof and the structure is adequate to resist any additional wind and gravity loads associated with the panels.

 

Access to the roof for inspection and maintenance must be considered when designing roof mounted structures. The roof cladding should be designed to resist regular roof traffic, or the installation of walkways is recommended.

Wires and conduits penetrating the cladding must be fitted with a gooseneck or proprietary cover to prevent reliance on sealant for weatherproofness and must be installed so that their base does not impede water flow.

 

4.10.3D Supported HVAC Installation

A well-designed HVAC installation allows free draining of water, isolates incompatible materials, has little reliance on sealant and avoids extensive dry-pan back flashings

 

Support structures must be constructed of materials compatible with the roofing material and separated from the roof by a non-absorbent and non-conductive rubber gasket. Typical proprietary support structures are manufactured from aluminium with stainless steel fasteners. Contact between the stainless fasteners and the roof cladding should be avoided, but run-off is acceptable. Contact with, or run-off from, copper onto painted and unpainted steel and aluminium roofs must be avoided.

 

4.10.3E Rib Supported Photovoltaic Bracket

Typical rib supported systems are Manufactured from aluminium.

 

 

4.10.3F Tilted Photovoltaic Installation

An example of a Proprietary tilted PV Installation.

 

 

 

4.10.4 Hot Water Runoff Corrosion 

The combination of hot water and copper is detrimental to all types of roofing, and hot water from copper pipes will exacerbate the corrosive effect of copper itself.

Water from the exhaust pipes of hot water cylinders or pressure relief-valves must not be permitted to discharge onto metal roofs. The copper pipes of solar panels, air-conditioning or other ancillary equipment must be sealed to avoid runoff onto any metal cladding or gutters other than copper.

4.10.5 Walkways 

Only aluminium, stainless steel, inert plastic, or hot-dipped galvanized steel framing members are acceptable for support structures above steel cladding and must be supported by the structural members or across the ribs of the profile adjacent to the ribs.  They should be designed to allow natural rain washing of the roof underneath.

4.10.6 Capillary Action 


While capillary action is both a durability and a weather tightness issue, it is considered here because it affects durability more often than water tightness, although the two are inter-related.

Capillary action is the ability of a liquid to flow into narrow spaces without the assistance of, and in opposition to, external forces such as gravity.  It is caused by the combination of intermolecular forces of surface tension in the water and adhesive forces between the liquid and surrounding surfaces.

The effect can be seen in the drawing up of liquids between the bristles of a paintbrush, in absorbent materials such as blotting paper or a sponge, in a burning candle, a fountain pen, or the cells of a tree.  The effect can occur in a tube, but also between two closely spaced mating surfaces.

 

 

Gravity will affect the degree of capillary action;  a low sloping pair of surfaces will attract liquid more by capillary action than a vertical surface, and a narrow tube will draw a liquid column higher than a wider tube.

Capillary action is an important consideration in cladding installation and design, and can be considered in four main areas:

  1. Closely stacked sheets of trapezoidal or corrugate profiles, or flat sheet that has the sheet ends exposed to rain, will draw water between the surfaces which can infiltrate a long way into the stack of material.  After a short time in the absence of air, it can form volatile corrosive products which are unsightly and detrimental to product life.
    While metallic coatings have temporary surface protection against wet storage stain and organic coatings also give some protection, there is no hard and fast rule as to how long this will last; it is up to the roofing contractor to take appropriate measures. Packs of close stacked sheets exposed to water must be fillet or cross stacked to allow natural air movement and drying, before the onset of wet storage stain.
  2. Capillary action can take place in the side lap of roofing sheets, or between a longitudinal flashing and the adjacent rib.  For this reason, side laps should be designed with a capillary break, and when calculating the water carrying capacity of a profile, the allowable water depth is taken as being to the bottom of the capillary bead, not the rib height. 
    Even corrugate profiles are designed to have an asymmetrical shape between under and over crest. However, this is not normally as effective as the capillary break on a rib profile.  This is one of the reasons why the minimum pitch for standard corrugate is 8°, although it has been proven to perform at lower pitches in short runs if the dimension of the overlap is not too generous and it is not extending into the water table.
    With longitudinal flashings, such as barges, the downturn into the pan should not be tight against the rib, but have a gap to avoid capillary action from occurring.
  3. Capillary action is also common between the end laps of sheets.  When short run sheets were the norm and end laps common,  the onset of corrosion normally occurred around the lap despite primer being applied to the surfaces.  It was most often concentrated on the upper end of the lap and was caused, not by rain water, but by condensation on the underside of the sheet entering the lap.
    End laps on roofing should be avoided where possible.  Where they do occur, end laps must be sealed at both ends to avoid ingress of moisture from both internal condensation and external rainfall, and end laps in vertical sheets must be sealed at the top end of the lap.

     

     

  4. The adhesive nature of water that causes capillary action can help drive water up the underside of the sheets at the eaves, rather than discharging into the gutter.  Therefore, the ends of all sheets laid to a fall less than 8° require a drip edge; and the minimum roof pitch of standard corrugate is 8°, as it is difficult to form a drip edge in that profile.The lower edge of all roofing sheets and flashings laid to a fall of less than 8° must be drip formed into the gutter to prevent capillary action. 

4.10.7 Crevice Corrosion/Wet Storage Stain 

Roofing materials exposed to the air react with the atmosphere to form a relatively stable surface. Exposing metals to water in the absence air causes the formation of unstable surface films.

Crevice corrosion occurs in crevices and confined spaces. Crevices are often created because of overlapping flashing or sheets of cladding, or between the sheets of close stacked materials.

Design details that trap moisture, dirt, and debris should be avoided.

Corrosion can appear even with a chemically neutral electrolyte. An example of this type of corrosion is the corrosion on metals underneath paint coatings and “white rust” — the wet storage stain on closely nested zinc coated roofing sheets. Other metals, such as aluminium/zinc coated and non-ferrous metals, can suffer similar damage.

 

 

Capillary action can cause white rust to occur throughout a length of cladding

 

 

White rust on galvanised sheets causes bulky white deposits to form that can quickly lead to red rust

If end-lapping of roof sheets cannot be avoided, both ends of the lap must be continuously sealed to ensure that neither condensation run-off from the under-surface nor rainwater run-off enters the lap.

Capillary action can cause water to be drawn into closely stacked sheets, resulting in crevice corrosion or wet storage stain on both metallic coated and non-ferrous materials. On metallic-coated steel sheets, the passivation coating gives some temporary protection against this process, as do organic coatings, but longevity cannot be guaranteed for the duration of this protection.  On non-ferrous, metals, wet storage stain can commence very rapidly.

Wet packs of sheets should be separated to allow surfaces to dry before substantial storage.

If wet storage stain appears on unpainted surfaces, the degree of erosion of the metallic surface may be slight despite the bulky appearance of the deposits. However, when left unchecked it, can quite quickly lead to substantial degradation.  If required, measurements can be taken of the thickness of the material or the metallic coating to determine the extent of erosion.

Even if the damage is superficial, the white deposits must be removed to allow exposure to the air to allow the normal formation of stable surface films.  Use a stiff bristle brush; wire brushes are not recommended as they will remove more of the protective coating.

4.10.8 Microcracking 

Microcracking is microscopic cracking on the surface.

  • Microcracking of the metallic and/or organic coating creates a crevice where the normal protection mechanisms of the coating are compromised. It can lead to premature corrosion failure.

The test requirement for paint adhesion in AS/NZS 2728 and by the NZMRM is a bend test over a nominated number of material thicknesses (T) and it is measured as the internal diameter.

The radius required to avoid microcracking on metallic coated and pre-painted steel is measured externally. To obtain an external radius, add two material thicknesses to the internal diameter, and divide the result by two.

 

4.10.8A Microcracking : a

0 T internal diameter = 1.0 T external radius

4.10.8B Microcracking : b

(b) 1 T internal diameter = 1.5 T external radius

4.10.8C Microcracking : c

(c) 2 T internal diameter = 2.0 T external radius

4.10.8D Microcracking : d

(d) 3 T internal diameter = 2.5 T external radius

4.10.8E Microcracking : e

(e) 6 T internal diameter = 4.0 T external radius

 

4.10.9 Gutter Leaf Guard 

Leaf guards are widely marketed as a way to prevent the build-up of vegetable matter in spouting. They may achieve this objective, but they often result in a build-up of a plant material poultice on the eaves line of the roof.

That can cause premature corrosion of the roof by chemical reaction, greatly increased time of wetness, and prevention of adequate ventilation of the underside of the cladding.

The COP recommends against the installation of such products. Alternative solutions include fitting a durable spouting material such as copper, installing rain heads with a leaf trap, or installing a proprietary leaf-proof spouting system.

 

 

4.11 Inert Catchment 

Run-off from inert surfaces such as glazed tiles, aluminium and aluminium-dominant metallic coatings, fibreglass, pre-coated metals, glass or any painted surface can cause corrosion of unpainted galvanised steel and other zinc-dominant metallic coatings. This is known as 'drip-spot corrosion' or inert catchment corrosion.

 

 

Water sitting on a surface absorbs carbon dioxide forming carbonic acid, which is reactive with zinc. On a galvanised surface, the carbonic acid reacts with the zinc and becomes neutral. On an inert surface discharging into an unprotected zinc surface, the carbonic acid is not neutralised, and reaction will be concentrated on the drip points of the inert surface onto the zinc surface.

As the formation of carbonic acid takes time to occur, inert catchment corrosion is normally seen at specific drip points of dew off a roof, rather than below rain washed painted walls and windows.

 

 

 

 

4.12 Ponding 

Ponding happens when water is unable to drain from a roof or gutter surface. Possible causes include lack of fall, poor penetration design, and damage to sheet ribs due to excessive spans or foot traffic. The accumulated water increases the time of wetness and can lead to poultice corrosion.

All paint systems on factory pre-painted materials are permeable to a degree and will delay, but not prevent, the corrosive effects of ponding.

Ponding can occur in gutters and spouting when joints or outlets are higher than the sole of the gutter, or when debris is left to accumulate.

To help prevent ponding, the minimum pitch for all metal roof cladding in New Zealand is set at 3°. At pitches of lesser pitches deflection of the structural members or settlement of the building can compromise drainage.

Low pitch roof spans must be sized according to the type and frequency of roof traffic to prevent ponding caused by rib damage, and penetration flashings must be of a free draining type.

4.13 Pitting Corrosion 

Pitting corrosion is a highly localised corrosive attack that forms pits which have a very small surface area, but which can be quite deep.

Pitting occurs on non-ferrous metal when the protective passive film breaks down, or where it has been weakened or damaged by contamination. When the break-through occurs in the passive film, the actively corroding pit constitutes the anode and the large passive film surrounding the pit acts as a cathodic surface.

The rate of dissolution of the metal is strongly influenced by the ratio between anode and cathode areas, consequently the "driving force" behind pitting attack can be very strong and deterioration can spread quickly.

4.14 Swarf Staining and Cut Edge Corrosion 

Swarf is the term given to the steel debris as a result of cutting or piercing a metal sheet or adjacent metal surfaces.

When cutting steel, any swarf remaining on the sheet starts corroding quickly and causes stains.  These stains are often mistaken for early deterioration of the cladding.

To some degree, swarf will normally be evident at the completion of any roof cladding job. The acceptability of swarf depends on how it got there, whether techniques have been applied to minimize it, and the visual exposure of the cladding.

 

Light, scattered swarf is acceptable in most situations.

 

Swarf created by acceptable means of cutting — i.e., power drills, self-drilling screws and shears — will be either loose or lightly adhered to the surface film of painted or unpainted sheets.  Most swarf can be removed by daily hosing, sweeping, or blowing which should be done at the end of each day and at the completion of the job.  Avoid blowing loose swarf under adjacent cover flashings.

Any remaining swarf will not be in contact with the metallic substrate and will not cause deterioration of the roof, its effect is aesthetic only.  Overly aggressive efforts to remove such swarf is likely to damage the appearance of the cladding without enhancing its durability

On highly visible surfaces, a soft rag and plastic spatula can be used to remove more tenacious swarf adhesions, or on painted surfaces, the use of a diluted mild household liquid cleaner might work.  Wire brushing, steel wool, or pot scouring cloths must not be used as they will damage the organic or metallic coating.

Swarf created by unacceptable practices, such as the use of grinders and friction power blades on, or adjacent to the cladding, is often hotter on contact with the cladding. The heat may cause it to embed deeply in the organic film and be in contact with the protective metallic substrate.

 

Friction cutting that creates swarf can also cause heat damage to metallic and organic protective surfaces.

This can severely affect the substrate; removal is difficult or impossible to achieve without mechanically damaging the decorative and/or protective coatings.

Swarf is not the only problem that cutting with friction blades can create. Such blades will often produce excessive heat at the cutting edge, which will degrade the organic and metallic coatings.

 

Often roof damage is caused by sub-trades accessing the roof after installation.  Roofers and other trades must be aware of how they are treating the material they are working on and the effect it may have on adjacent surfaces.

Where work is done above or adjacent to an installed roof surface, or where the roof is used a work platform for subsequent work, the main contractor must take steps to make sure the existing roof remains undamaged.

 

 

This roof shows evidence of mechanical damage to the coating, rib traffic damage to adjacent ribs within a purlin span, and excessive swarf by unacceptable cutting practices. In this case the only logical remedy was replacement.

4.15 Clearances 

To ensure the edge of the flashing does not mechanically remove protective coatings on the cladding, there must be enough clearance between the edge of a vertical flashing, or a notched flashing, and the cladding. Similarly, the edges of cladding running parallel to flashings, such as at a window head, should have clearance to avoid mechanical damage and allow drainage.

Having the lower edges of flashings apart from the surface they are covering helps to improve the cut edge durability of the flashing. Kick-out barge details are preferred to bird’s beak barge details for the same reason. The size of the clearance is not critical, but typically it is more than 5 mm.

 

 

4.15.1 Ground Clearance 

Clearance is required between the bottom of profiled metal cladding and large flat surfaces. For timber-framed dwellings, E2/AS1 requires a clearance of 35 mm to an adjacent roof, 100 mm to paved ground, and 175 mm to unpaved ground.

The clearance requirements for unlined buildings is less than that required for lined buildings, as the absence of lining enables the inner face of the cladding to dry more rapidly, and inspection and maintenance of the framing can be practically achieved.

4.15.1B Importance Levels from NZS 3604;2011 (Table 1.1)

Level 1Structures presenting a low degree of hazard to life and other property
Level 2Normal structures and structures not in other importance levels
Level 3Structures that may contain people in crowds or contents of high value to the community, or may prose risks to people in crowds.
Level 4Structures with special post-disaster functions.

4.15.1C Minimum Ground Clearance for Lined Buildings

Minimum Design Ground Clearance for
Profiled Metal Cladding on Lined Buildings of Importance Level 2.
 
Ground TypeMinimum Clearance
Garage door opening25 mm
Walls under canopies35 mm
Paved100 mm
Unpaved gravel125 mm
Unpaved lawn150 mm
Unpaved pasture175 mm

 

Importance level 1 buildings may have a lesser clearance provided occupant maintenance prevents the build-up of debris against the cladding.

Greater clearance may be required where gardens abut a wall, where lawn grasses are not grazed or maintained, or where soil spillage from adjacent banks may occur. Future landscaping effects on ground levels must also be considered.

4.15.2 Site Management 

The effectiveness of clearances in achieving durability requirements is subject to the occupant ensuring that vegetation, debris, and soil do not build up against the cladding surface. Design clearance from a surface is no guarantee of durability as effective clearances are subject to site development, occupant behaviour and building maintenance.

4.15.2A Cladding Open to Air
 

Cladding which is open to air will experience the normal wet/dry cycles for which it is designed.

4.15.2B Vegetation in Contact with Cladding

Vegetation or earth in contact with the cladding will increase the time of wetness and may contain corrosive compounds.

The separation of profiled metal claddings from corrosive surfaces such as wet timber or concrete is more critical at the bottom end of cladding, where high humidity levels may be experienced for extended periods. This may take the form of a 3 to 6 mm gap, an inert self-adhesive tape or a PVC vermin strip.

Internal environments are also important, ventilation must be adequate for the building use, and absorptive of corrosive substances must not be in prolonged contact with the external or internal face of the cladding or structure.

4.15.2C The Result of Debris Build-up Against Cladding

 
Allowing build-up of material against wall cladding can result in corrosion regardless of nominal ground clearance.

4.16 Materials 

Metals used in the roof and wall cladding industry in New Zealand are:

  • steel coated with zinc - Galvanised steel;
  • steel coated with an alloy of aluminium and zinc, sometimes with the inclusion of other minority elements;
  • aluminium;
  • copper;
  • zinc;
  • stainless steel; and
  • lead.

*Many of these can coated with an organic coating, including acrylic, polyester and PVDF.

4.16.1 Steel 

4.16.1.1 Metallic Coatings 

For most of the nearly 200-year history of lightweight steel cladding, the protective metal coating has been made from zinc (usually with minor additions of other metals), and this is called galvanised steel. It works by the zinc sacrificing preferentially to the steel.

In the second half of the 20th century research looked for metallic coatings which would provide longer life. Aluminium was tried as a coating material because of its passive surface, but it was not satisfactory on its own. However, aluminium alloyed with zinc and other metals produced more corrosion proof products than any metal on its own. (See 4.4.2 Barrier Protection.)

We now have two groups of metallic coatings for steel cladding products — zinc-dominant coatings, which primarily provide sacrificial protection; and aluminium-dominant coatings, which primarily provide a barrier protective coating of aluminium oxide. Coatings containing both aluminium and zinc are now the preferred coating for roof and wall cladding products, although zinc-based coatings continue to predominate for various other products.

The composition and weights of these coatings are described in detail in AS 1397:2011. The following sections discuss metallic coatings in the order in which they appear in AS 1397, not their rate of use in the market.

4.16.1.1.1 Coating Thickness 

Steel was zinc-coated for many years by dipping short lengths of flat or profiled sheet metal in a bath of molten zinc, and the steel was then hung to cool while the excess zinc coating drained off.

More than sixty years ago manufacturers developed a continuous hot dipping method. During the continuous hot dipping process, the steel coil is run through a bath of molten metal. The thickness is controlled by blowing off the excess coating with air jets applied to both sides of the strip as it leaves the molten metal bath. 

Continuous hot dipping, as opposed to the batch immersion process, is more cost-effective and allows for greater control of the consistency, thickness, and surface condition of the metallic coating.

It is a similar process to that for continuous paint coating, shown in 4.17.1.1 The Paintline Process, with priming, coating, and ovens replaced by the molten metal tank and blow-off section.

The atmospheric corrosion performance of a hot-dipped zinc coating is closely proportional to its thickness.

The thickness of coatings in micrometres (µm) can be measured with a non-destructive magnetic induction meter or similar device which can then be converted into grams per square metre (g/m²).

 

 

There is confusion about the method of describing the coating thickness of coil-coated sheet and strip products in g/m², compared to products that were hot-dipped after fabrication. The coating thickness of sheet and strip refers to the collective amount of coating on both sides of the sheet, effectively dividing the coating weight by half. It is invalid to equate the coating weight in g/m² of hot-dipped zinc coatings on fabricated products, such as nails and screws, with that of metallic coatings on sheet and coil; the coating thickness of the fabricated products relates to one surface only.

A micron (µm) is one-thousandth of a millimetre.

4.16.1.2 Galvanised Steel 

Zinc Coating, commonly called galvanising, is still one of the most common metallic coating processes for steel. Galvanising describes various methods of adding a metallic zinc coating to steel to give it cathodic protection; also known as galvanic protection.

Galvanised steel is classed as a “Z”-coating and has a bold crystalline pattern or spangle, a random geometric pattern that resembles frost on a window.

There are many processes for galvanising, but only products dipped or immersed in a bath of molten zinc can be called hot-dipped galvanised, the process used for the metallic coating of steel roof and wall cladding.

The thickness of the coating can be more precisely controlled on a continuous coil galvanising-line than it can be with other methods.

The standard coating weight for unpainted galvanised coil and sheet used for roof and wall cladding is 450 g/m², designated Z450, but other coating weights are available. The coating weight for products intended for painting is 275g/m², and it is designated Z275.

Since the advent of ZM coatings, minimised spangle zinc coated products, typically used for painting, are now designated with the “M” after the weight, e.g., Z275M.

The process of zinc coating by electro-plating gives a much thinner protective film and is not considered suitable for painted or unpainted cladding materials exposed to the weather.

4.16.1.3 ZA Coatings 

ZA coatings are a zinc-aluminium alloy coating consisting of 95 % zinc and 5% aluminium by mass, with the addition of lanthanides.  It is commonly known as Galfan® and is as designated ZA in AS/NZS 1397. As an European product it generally conforms to EN 10214..

ZA serves the same purpose as galvanised Z and AZ coatings, but has different corrosion characteristics than both.

ZA coatings are not currently available in NZ or Australia.

4.16.1.4 ZM Coatings 

ZM coatings are zinc-aluminium alloy coatings with a majority of zinc and a small amount of magnesium. Steel with a continuously hot-dipped coating of zinc with 5 -13% aluminium and 2 - 4% magnesium is designated in AS 1397:2011 as ZM. In New Zealand, it is commonly marketed as ZAM.

The coating weights are similar to Zinc coatings, with ZM 240 used for products which will later be coil coated and ZM 450 for unpainted products.

Unpainted ZM products have been used for roofing accessory and rainwater cladding applications in New Zealand, but are more commonly found in factory pre-painted products.

4.16.1.5 AZ Coatings 

AZ coatings are zinc-aluminium alloy coatings with a majority of aluminium. AZ coating, marketed as Zincalume® steel, is an alloy of zinc and aluminium which is now the most commonly used coating in New Zealand for protecting steel roof and wall cladding.

AZ coating is applied in the same way as other coatings, but with a pot temperature at about 140˚C higher than galvanised coating, and it is rapidly cooled to provide a dual-phase microstructure.

The alloy consists of 50 to 60% aluminium, zinc, and a small addition of silicon. In New Zealand, the ratio is nominally 55:45. These percentage ratios are by mass; by volume, the percentage ratio changes to approximately 80% aluminium and 20% zinc. Volume is probably a more realistic measure of its nature.

The alloy coating thickness generally used for steel roof and wall cladding is 150 g/m² (AZ150). This coating is approximately the same thickness (0.04 mm) as Z275 zinc. AZ200 coatings are available as a substrate for organic coated products to be used in very severe environments.

An AZ coating protects steel both as a barrier and sacrificially, as the aluminium content provides a barrier, while the zinc content of the coating will sacrifice itself to protect the base steel.

The AZ coating is finer grained than zinc alone and has a silver matt hue with a lightly visible spangle. This finish has a relatively high level of initial reflectivity, which darkens over time.

A thin acrylic film is applied during manufacture in New Zealand. The acrylic film acts as a roll forming lubricant and minimises finger marking and surface discolouration.

4.16.1.6 AM Coatings 

Adding magnesium to an aluminium dominant zinc-aluminium alloy coating improves the cut edge corrosion resistance to a similar level as zinc coating, but still confers the improved surface protection and slower erosion rate of AZ coatings.

Steel with a continuously hot-dipped coating of 47 - 57% aluminium and zinc, with the addition of 1 - 3% magnesium by mass is designated in AS 1397:2011 as AM. It is not currently sold in NZ but is under test.

Coating weights may be AM100, AM 125 or AM150.

4.16.2 Stainless Steel 

 

Stainless steel is a durable, corrosion-resistant material used in harsh environments when a non-weathering finish is desired. Chromium forms a tenacious oxide protective film on stainless steel that is transparent and self-healing, as it will repair itself on exposure to the atmosphere.

Stainless steels are resistant to most chemicals, but are subject to crevice and pit corrosion (see Wet Storage).

Some light surface staining known as tea staining may appear, but it is not damaging to the product.

Most stainless steel roof and wall cladding, flashings and panels in New Zealand are made from the 300 series of austenitic non-magnetic stainless steel, which contain chromium, nickel, and manganese, with 304 and 316 being the most common grades.

Grade 304 stainless steel is an alloy of 18% chromium and 8% nickel that provides high corrosion resistance and is known as an all-purpose alloy.

Grade 316 stainless steel should be specified where tea staining must be avoided. It contains 16% chromium, 10% nickel, with 2% molybdenum added, which increases resistance to staining and corrosion.

Grade 445 ferritic stainless steel is now available in New Zealand, which combines the corrosion resistance of grade 316 with formability approaching that of carbon steel. As the work hardening of 445 is much lower than with austenitic grades, it can be formed in a similar way to carbon steel and is more easily sheared.

Grade 445 stainless steel contains 22% chromium and 1.2% molybdenum and no nickel. It has lower thermal expansion than other grades, so it is less likely to distort in the heat of the sun. The yield stress and hardness of 445 is higher than 304 and 316, but the tensile strength and elongation properties are lower.

The corrosion resistance grade of 445 is similar to grade 316 in most marine and aggressive industrial environments.

Stainless steel is available in various mill finishes from dull matt to highly polished. The most common finishes for roof cladding and sheet metal flashings, are those designated as 2B and 2D.

The 2B finish is a bright, cold-rolled finish that is highly reflective and 2D is a dull finish that is less reflective. BA is a bright reflective surface only suitable for decorative cladding in thicker gauges. Embossed patterns are available that reduce visible distortion and minimise glare and reflection.

Stainless steel should not be cleaned with steel wool, but stainless steel wool or synthetic abrasive pads can be used. Cleaning should be done with care as roughening the surface may promote further stains.

Stainless steel fixings should be used with stainless steel sheet to avoid dissimilar metal corrosion. The fastener grade must match the grade of the cladding.

There is no well-defined yield point for stainless steels. Fully annealed or standard annealed tempers are used for ease of forming with 304 and 316 having an approximate yield strength of 290 mPa.

Austenitic stainless steels require different forming techniques than other metals, and are known to be tougher and more difficult to form than carbon steel of the same thickness, e.g.,m when shearing stainless steel the equipment capacity should be increased between 30% - 50%. Because of the toughness of the metal, sharp cutting edges dull more quickly than when used with carbon steel.

Although stainless steel is not much harder than mild steel, increased power is necessary to form it because of its high ultimate strength and its higher work hardening rate. As most forming machines are rated for the heaviest gauge steel this capacity should be de-rated by 40%.

Precautions should be taken not to contaminate the surface of the metal by inclusions from roll forming or folding equipment. It can appear as rust spots on stainless steel, which is detrimental to performance. Stainless steel coil and sheet can be supplied with a strippable film on both faces to avoid this contamination.

4.16.3 Aluminium 

The aluminium alloys used in New Zealand for roof and wall cladding are included in the 5000 series.

  • Aluminium 5005 has excellent workability, weldability and corrosion resistance.
  • Aluminium 5052 is a higher strength marine grade alloy with exceptional resistance to corrosion in marine or industrial environments.

Following strain-hardening of aluminium alloys, tempering increases the ductility by low-temperature heating, and their description regarding hardness relates to the last number, e.g. H12 or H32.

The description of tempers given to aluminium alloys can be confusing because the different alloys are strain hardened in different ways. As a result, different alloys with the same hardness description may have significantly different yield strengths.

 

Pure aluminium (99%) can be used as a soft edging for ridge or apron flashings required to act as a wind barrier.

Aluminium alloys are available in three surface finishes.

  • Mill finish: A smooth, lustrous finish which will dull relatively quickly.
  • Stucco Finish: An embossed mill finish, which reduces the specular reflectance of mill finish sheet.
  • Painted Finish: A range of painted finishes are available similar to those offered in painted steel.

The high reflectance and emissivity of unpainted aluminium can reduce heat transmission considerably.

Aluminium develops a thin oxide film on the surface that is impermeable to most airborne contaminants, except for strong alkalis and acids.

4.16.3A Aluminium Hardness End-use

Note: Typical stocking of 5052 alloy, H36 allows for rollforming both corrugate and trapezoidal profiles. Trough section may require H34 material. H36 material can be used to manufacture most flashings, except those requiring soft edging or hemming.
Alloy;Yield MinimumTypical Use
5005 – H32 Quarter Hard85Lockseam
5005 – H34 Half Hard105Folding
5052 – H32 Quarter Hard160Lockseam
5052 – H34 Half Hard180Folding and curving
5052 – H36 Three Quarters Hard200Rollforming and folding
5052 – Fully Hard220220Rollforming

 

4.16.4 Zinc 

Zinc is a traditional roof cladding material and weathers to a dark grey patina after environmental exposure, like galvanised steel, except there is no spangle effect on the surface. Zinc roof panels and flashings are commonly 0.7 mm thick, although heavier gauges can be used. Zinc roofs are usually fully supported on sarking.

The staining potential of zinc run-off onto other surfaces is less than that of copper and lead. Flat zinc panels must be adequately vented from underneath and are available with a high-build lacquer coating to help prevent corrosion of the under-surface.

Zinc has approximately twice the thermal expansion coefficient as steel, so allowance for expansion must be made accordingly.

Under 7 °C the metal becomes brittle and is difficult to form without fracturing.

Zinc used for roof cladding generally contains small percentages of titanium and copper, which add to the properties of pure zinc.

Zinc is also available in a range of pre-patinated surfaces.

4.16.5 Copper 

Copper is a naturally durable product.

Copper grades 122, 110, and 102 may be used in construction. Grade 122 is the most commonly used; it has been deoxidized with phosphorus, which makes it weldable. The other grades cannot be welded but can be soldered.

Copper darkens in reaction to the atmosphere. Near seawater or industrial sources of sulphur-containing gases, a green patina may develop in time. In different environments, the weathered colour may vary from dark brown to almost black.

Copper-containing alloys, such as brass and bronze, are available when different colours are requested or for accessories requiring greater strength.

Copper is more malleable than steel sheets, and annealed copper is used for hand folding or where a high degree of formability is required. Roll formed roofing, wall cladding, gutters, spouts, and flashings are typically made from half hard copper. Copper roofs are normally fully supported on sarking.

Copper must be protected from contamination when being processed with tools that have been used to process other metals because the resulting inclusions might cause pit corrosion.

Neither copper nor run-off from copper should come in contact with less noble metals, as it will cause galvanic corrosion. Avoid installing copper in contact with or receiving run-off from bituminous material or other acidic surfaces, because it prevents the formation of the protective patina, causing discolouration and a shortened lifespan.

4.16.6 Lead 

Historically, lead has been a popular choice for roof cladding and flashings, because it is naturally durable and is easily shaped using hand tools at ambient temperatures, without the need for softening or annealing.

Lead has an inherent lack of mechanical strength and is laid on solid sarking. It has high thermal movement and, over time, there is a risk of distortion and lead sheet cracking. Sheet lead is available in weights from 6 kg/m² to 40 kg/m².

The thinner the lead, the shorter the length should be. A maximum length of 1500 mm or less than 1.5 m² is ideal.

Run-off from a new lead roof can stain other metals with a white lead carbonate. Application of a proprietary product or boiled linseed oil and mineral turpentine mixture can avoid that happening.

A factory applied cured coating that inhibits the contact between lead and oxides with water is available. The lack of contact reduces the potential for run-off staining other metals, and of lead entering ground water systems. Avoid using lead roofs to collect potable water.

4.16.7 Translucent Sheeting 

Translucent sheeting should be manufactured from naturally durable products or have a protective surface film to avoid ultra-violet degradation. (See Natural light for more information.)

GRP

4.16.8 GRP 

GRP is a composite material made up of polyester resin, reinforced with glass fibres.  It is protected from UV erosion by a surface coating consisting of a gel or laminate.  The composite is extruded and set over forming moulds to match specific roofing profiles. 

GRP can often be used as translucent sheeting or it can be supplied with a clear, or opaque gel coat to the weather surface to provide a high level of corrosion resistance to aggressive atmospheres, where coated metal or even non-ferrous metals may not perform as required.

Examples of use can be found in extreme environments such as wool scouring plants, fertilizer stores, tanneries, acid plants and smelters, abattoirs, compost plants, galvanizing plants, and buildings in harsh geothermal areas.

Where an entire roof is clad in GRP, (rather than individual sheets separated by metal sheets as in a typical case of translucent sheeting), it affects the trafficability and safety requirements. If a GRP roof is required to be accessible, it can be manufactured incorporating woven roving reinforcement into the resin matrix to make it trafficable. Another option would be to install stainless steel safety mesh under the roof cladding, if required.

4.17 Organic Coating 

4.17.1 Pre-painted Factory Finish 

The performance of metallic coated profiled metal can be enhanced by the application of an organic (“paint”) coating.  In pre-painted steel coil this is applied continuously as a two-part primer/topcoat system, prior to the material being roll formed into profile.

The different combinations of metallic coating/primer/topcoat all have different characteristics and must be matched to the material and the environment in which they are to be exposed.

The primer and the topcoat have different performance requirements to fulfil.

4.17.1.2 Primer 

The purpose of the primer is to adhere to both the substrate and the topcoat, and to give added corrosion protection.  Primers used on coated steel coil have anti-corrosive pigments which inhibits corrosion through an electro-chemical reaction.

4.17.1.3 Top Coat and Backer 

This is the outer skin and it must give the desired appearance. In terms of durability, it provides a measure of barrier coating while still being breathable, and prevents UV degradation of the primer.

Functionally the top coat must be hard enough to prevent excessive marring during profiling and installation; and when in use, it must:

  • be flexible enough to form to relatively tight bends without excessive micro-cracking;
  • be resistant to fade in NZ’s harsh environment;
  • withstand extremes of temperature; and
  • be a suitable surface for the collection of potable water.

More recent innovations include solar reflectance technology to minimise the amount of solar heat gain gathered by the cladding

Backer coats generally have the same primer and a thinner top coat than the upper surface. Double-sided systems can be specified for areas where the underside is seen, but in external environments these will be exposed to salt spray and other contaminants, and it cannot be assumed that the underside will last as long as the rain washed top surface — even with regular maintenance.

As paint formulations from different suppliers may have different performance characteristics, it is important that cladding and accessories are supplied from the same manufacturer as differing weathering characteristics may result in visible variance in appearance.

Surface coating must comply with AS/NZS 2728. Cladding and flashings must come from the same source and have the same coating specification, so that fade rates are similar. Fade rates must not exceed those stipulated in AS/NZS 2728. All coatings must be lead free and suitable for the collection of drinking water.

AS/NZS 2728 has been deemed ambiguous in that it can be interpreted as allowing accelerated testing to determine colour fastness and durability. Such tests have been found to be an unreliable indication of a system’s performance in real-life situations. MRM has therefore adopted an interpretation of this standard as the compliance standard for its members.  In the MRM standard, the four-year real-life testing for durability and colour fastness are clarified as being Normative (Compulsory).

4.17.2 Post-Painted Factory Applied finish 

This coating system is most commonly used for pressed metal tiles.

Where a coating is applied to a Metallic coated steel or Aluminium substrate after the profile of the roofing sheet has been formed is referred to as a Post-Painted Factory Applied Finish. The metallic substrate may have a coil coated primer, coil coated anti-finger print coating, a factory applied post painted primer or be cleaned and treated to suit the application of the coatings in a factory environment.

Post Painted coating applied to the substrate vary from smooth matt or gloss painted finishes and textured granule finishes bonded to the substrate with high build coatings. The dry film builds are high compared to pre-primed or coil coated products and the films seal any micro-cracking of the metallic coating that may have occurred in the forming process.

The corrosion performance of the post painted factory applied finished products is influenced by the substrate. The use of Aluminium Zinc metallic coating (AZ) has been proven to work extremely well with the post painted finishes in almost all environments.

The coatings are durable with expected first time recoating maintenance of over 15 years. Touch up of these coatings is possible as the same coating formulation used can be applied in the field. As with any organic coating there will be a gradual breakdown of the resin systems which may result in chalking of the surface.

Chalking is the result of erosion of the surface coating and slight colour changes may occur as the surface resin is eroded. The extent of change depends on pigments used and also the gloss level, a coating that started out as a matt finish will change very little while a gloss finish will appear to have changed more.

Granule-textured finishes use crushed rock that is either natural or natural rock with the surface coated with a ceramic coating that incorporates light-fast inorganic pigments. Both the natural rock and ceramic coated granules provide exceptional long term colour durability. The coatings used to adhere the granules to the substrate are designed to be flexible in all environments and, although durable, the coating is protected from UV by the UV opaque granules.

A clear coating is applied to the granules during the coating process, which helps bond the granules into the adhesive base coating and provides a robust surface reducing any damage during the transportation and installation process. As this clear coat weathers, it exposes the natural or coated surface of the granules, which is slightly duller than the initial finish.

4.17.3 Powder Coating 

Powder coating is generally used to colour match accessories used with pre-painted steel cladding and rainwater goods. The use of powder coating on metallic coated steel roof and wall cladding and flashings is not recommended for the following reasons.

  • It will fade at a noticeably different rate to adjacent pre-painted metal surfaces.
  • Powder coated products are length limited by the size of the curing oven and cannot be re-formed after coating.
  • Drilling or cutting of the sheet after powder coating will create an exposed edge vulnerable to under-cut corrosion.
  • Standard powder coating does not have the corrosion resistance of pre-painted metal and is vulnerable to post coating damage and edge creep corrosion.
  • It is difficult to obtain adhesion to powder coated surfaces when overpainting, and the remaining powder coat surface may not provide a good substrate for such.

Power coating may be used to colour match accessories but cannot be relied upon to increase durability, unless specific pre-treatment and powder coating systems are specified and applied.

4.17.4 Lap Priming 

End laps should be avoided where possible. Side laps on profiled sheets do not require priming as they are designed with capillary grooves to drain naturally, or other means to avoid the accumulation of condensation or rainwater.

4.18 Accessories 

4.18.1 Fastener Durability 

All cladding fasteners must be compatible with the material, suitable for the environment and a durability equivalent to that of the cladding material. All exposed fasteners must have a minimum durability of Class 4. (See 17 Testing)

Only aluminium or stainless steel screws and washers should be used on pre-painted aluminium roof and wall cladding. Stainless steel fasteners must not be allowed to come into contact with the cladding and should be installed through oversize holes.

Sealing washers must be non-conductive to prevent electrical contact between the screw, the metal washer and the cladding surface.

Steel cladding screws can be subject to hydrogen embrittlement when they are hot dipped galvanised. Alternative methods of galvanising, such as peen plating and other metallic coatings, are generally used in combination with an organic coating.

Care should be taken to minimise damage to the head of the nail or screw when using colour matched painted hot-dipped galvanised nails, bolts and screws.

Sandblasting in exposed conditions can significantly reduce the coating thickness and the longevity of the fastener.

Premature failure can result when the shanks of the screws and the eaves purlin are exposed to sea spray, and a high-fronted gutter is recommended to help prevent this.

The performance of the shank of the fastener is also affected by internal environments when the contaminant is inside the building, e.g., animal shelters, fertiliser works.

 All fasteners should be easily identified by a code stamped on the head to identify the manufacturer and the coating class.

4.18.2 Screw Guns 

To avoid damage to the coating to the screw-head, drivers should not be of the impact type and should be fitted with a snug fitting driver bit.

4.18.3 Sealant 

Sealant should be neutral cure silicone or MS polymer.

4.18.4 Underlay 

Underlay should have durability no less than the cladding material, and be compatible in contact with the roofing material.

4.18.5 Underlay Support 

Galvanised wire netting or mesh can be used to support underlay where the internal environment is not aggressive, but is not to be used with painted aluminium.

Plastic mesh, tape or string may be used to give support at a maximum of 300 mm centres. These alternatives should be used when underlay support is required with aluminium cladding. Ensure that the underlay-support fasteners do not come in contact with the underside of the cladding.

For buildings with harsh internal environments, stainless steel, or PVC-coated mesh and netting are available.  PVC-coated mesh may suffer degradation from UV radiation if used externally or when it is exposed to high levels of direct or reflected sunlight.